The present invention is directed to fault tolerant liquid quantity measurement systems, in general, and more particularly, to fault tolerant liquid quantity measurement system using multiple model state estimators.
Liquid quantity measurement systems, like an aircraft fuel gauging system, for example, may be physically compartmentalized into tanks remotely located from each other and each tank may have compartments or sections referred to as bays. The tanks and bays of the system may encounter significantly different environmental and operational conditions, such as temperature, vibration, aeration, attitude and the like. To provide accurate liquid quantity measurements under these conditions, each tank, and possibly even each bay thereof, may be instrumented with its own set of sensors to measure the liquid parameters affected by these conditions. Accordingly, the liquid quantity of each tank and/or bay may be independently computed utilizing the corresponding set of sensors in each case, and the total quantity may be computed as the sum of the individual quantities. With the total amount of sensors used in the overall liquid quantity system, it is of paramount importance to include a level of fault tolerance in the system to maintain reliability for flight critical operations. While the present systems are considered reliable, there is always room for improvement.
Current systems offer some degree of fault tolerance by including built-in-test (BIT) circuitry in the interface modules which receive and signal condition the various sensor signals for a tank and/or bay thereof. In addition, the sensors may be divided redundantly for inputing to separate interface modules for added tolerance to a fault in an interface module, for example. Still further, presently proposed systems offer sensor fusion techniques for computing liquid quantity. Some sensor fusion techniques are considered to offer built-in fault tolerance without explicit fault detection. Competitive sensor fusion techniques, for example, operate with physical sensor redundancy, whereby a number of homogeneous sensors measure the same physical quantity, and simple algorithms, such as averaging, trimmed averaging, rank selection filtering or other voting schemes may be used to provide fault tolerance without explicit fault detection. In order for these systems to work, more than half of the sensors must function properly so they can form a majority. For example, in order to detect an accelerometer sensor failure through competitive fusion techniques, it is necessary to use at least three separate accelerometers.
Other sensor fusion techniques use analytical redundancy instead of physical redundancy whereby the sensors measure parameters that depend on each other via a set of equations. Thus, the outputs of each sensor may be predicted from the outputs of the other sensors. In these type systems, fault tolerance is achieved via appropriate partitioning of the sensor set. For example, suppose that the fuel mass of an aircraft were calculated by a number of algorithms that use different subsets of sensors, this would result in a set of fuel mass estimates. Then, trimmed averaging, median filtering, or other voting scheme could be used to reject or discount outliers caused by a sensor failure. But, this technique requires that for any single sensor fault, more than half of the competing algorithms remain unaffected.
Accordingly, such sensor fusion techniques as herein above described alone will not render the system single-fault tolerant, and there is still a need for a separate fault detection module. The present invention is directed to a fault tolerant system which overcomes the drawbacks of the aforementioned systems and offers an implicit sensor diagnosis performed in conjunction with the sensor fusion process itself.
In accordance with one aspect of the present invention, a fault tolerant liquid measurement system comprises: a plurality of sensors for measuring parameters of a liquid in a container; each sensor generating a measurement signal representative of the liquid parameter measured thereby; means for grouping said sensors into a number of sets, each set including some sensors of another set; and processing means for processing the measurement signals of each set of sensors to determine for each set of sensors a first estimate signal representative of a likelihood of measurement signal validity for the measurement signals of the corresponding set, and a second estimate signal representative of liquid quantity in the container based on the measurement signals of the corresponding set, said processing means for determining a third estimate signal of liquid quantity in the container based on a function of the first and second estimate signals.
More specifically, the fault tolerant liquid measurement system comprises: a number M of sensors for measuring the parameters of the liquid; means for grouping the M sensors into M+1 subsets, one subset including all M sensors and said other M subsets each including all of the M sensors except one and said missing sensor in each of said other M subsets is a different sensor of said M sensors; and processing means for processing the measurement signals of each subset of sensors with a respectively corresponding algorithm based on a state estimation model to determine for each subset of sensors a first estimate signal representative of a likelihood of measurement signal validity for the measurement signals of the corresponding subset, and a second estimate signal representative of the liquid quantity based on the measurement signals of the corresponding subset; the processing means for determining a third estimate signal of the liquid quantity based on a function of said first and second estimate signals.
In accordance with another aspect of the present invention, a system for detecting a faulty measurement signal comprises: means for grouping the sensors into a number of sets, each set including some sensors of another set; and processing means for processing the measurement signals of each set of sensors to determine for each set of sensors a signal representative of a likelihood that at least one of said measurement signals of the corresponding set is a faulty measurement signal, said processing means for detecting a faulty measurement signal of liquid quantity in said container based on a function of said likelihood signals.